fix: 修复 golangci-lint depguard 和 gofmt 错误

将 redis leader lock 逻辑从 service 层抽取为 LeaderLocker 接口，
实现移至 repository 层，消除 service 层对 redis 的直接依赖。

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/service/scheduled_test_port.go

@@ -59,3 +59,10 @@ type ScheduledTestResultRepository interface {
 	ListByPlanID(ctx context.Context, planID int64, limit int) ([]*ScheduledTestResult, error)
 	PruneOldResults(ctx context.Context, planID int64, keepCount int) error
 }
+
+// LeaderLocker provides distributed leader election for background runners.
+// TryAcquire attempts to acquire a named lock and returns a release function
+// and true if successful, or nil and false if the lock is held by another instance.
+type LeaderLocker interface {
+	TryAcquire(ctx context.Context, key string, ttl time.Duration) (release func(), ok bool)
+}